It is a very tough language for non-Balinese to master, and while related to Indonesian, is not mutually intelligible with it. You are unlikely to need to learn Balinese, as virtually every Balinese speaker is also able to speak Indonesian, and many can even manage some English. That being said, attempting to communicate in Balinese would almost certainly please the locals.

The language has three main registers depending on the status of those speaking:

low (basa ketah)

middle (basa madia)

high (basa singgih)

This phrasebook will cover the low register only as that is the language spoken by the vast majority of Balinese, and the only one likely to be encountered by visitors.